Surface processing



United States Patent SURFACE PROCESSING "Frederic Koury, Lexington,Mass., assignor to Sylvania Electric Products Inc., a corporation ofMassachusetts 12 Claims. (Cl. 4142) The present invention relates to themanufacture of germanium translating and transducing devices, andparticularly to reagents and etching methods for preparing suchdevices,and to the resulting germanium translators. This application is adivision of my earlier filed application Serial No. 106,493 filed July23, 1949, now abandoned.

In a customery procedure for preparing rectifying-contact semiconductordevices, purified germanium is melted and slowly cooled to promotecrystallization and a suitable doping constituent is included. Theresulting ingot is sliced into slabs with a diamond wheel, and polished,a procedure providing a true, fiat surface against which one or moreresilient whiskers, individually constituting rectifying contacts, areultimately pressed to constitute a semiconductor translator. Themechanically-worked germanium on the surface of the slab should beremoved for best electrical characteristics.

Heretofore, it was commonplace to surface-treat crystalline germanium byan aqueous etching bath, including hydrofluoric acid, nitric acid, and asmall quantity of cupric nitrate. The function of the cupric nitrate issomewhat obscure, but it appears to be necessary for the effectiveaction of the HF-HNO3 bath. The maximum voltage that acceptablerectifiers will withstand in the inverse or nominal non-conductingdirection and the inverse resistance of rectifiers using germaniumetched as described, leaves much to be desired.

Accordingly, it is an object of the invention to provide improvedreagents for surface-treating crystalline germanium in order to preparethe germanium for incorporation into a crystal translator.

A further object of the invention is to provide a substitute reagent andprocess for etching germanium.

Certain of these objects are realized by using hydrogen peroxide withhydrofluoric acid as disclosed and claimed in my earlier filedapplication. The mechanism involved in the etching of germanium by meansof the novel HF-HzOz bath is not a simple one. Water itself is known todissolve certain physical forms of germanium oxide. However, it has beenfound that each constituent separately in an aqueous bath is inoperativeto produce acceptable rectifying surfaces, but with the combination ofthe hydrofluoric acid and the hydrogen peroxide, a surface is providedwhich, with a suitable contact, yield devices of excellent rectifyingproperties.

Although the above form of etching bath represents a notable achievementin the surface treatment of germanium preparatory to fabrication intorectifying-contact semi-conductor devices, there is an extremeindustrial hazard incident to the use of hydrofluoric acid. It is knownto be an extreme corrosive, especially when persons applying the processmay be exposed to the reagents.

The present application is concerned specifically with the formdisclosed in my aforesaid earlier filed application relating to anetching bath for germanium, which includes hydrogen peroxide andalkali-metal hydroxides, including calcium hydroxide, potassiumhydroxide, and

Patented Apr. 3, 1956 2 sodium hydroxide, and salts of these metalsthat-decompose in water to yield the appropriate hydroxides.(Specifi'cally, it is my firm belief that in the hydrogenperoxideand-alkali metal hydroxide bath, thehydrogen peroxide is apowerful oxidizing agent which reacts withgermanium to produce itsoxide, while the alkali-metal hydroxide acts as a solvent for thegermanium oxide, and in this way removes objectionable mechanicallydeformed"me tailic germanium from the crystalline germanium surface.

In accordance with this aspect of the invention, there is provided animproved etchentfor surface-treating crystalline germanium, minimizingtherisk of industrialgliaz ard, such as the damage and injury to humanskin; incident to exposure to HF.

As a further feature of the present invention; starting materials,including peroxides of sodium, andpotassium, are hydrolized to yield thecorresponding hydroxides and hydrogen peroxide in aqueous solution. Thesolution products of these peroxides, as well as aqueous solutions ofalkali metal hydroxides plus hydrogen peroxide, result in high yields ofexcellent semiconductor units.

A presently preferred process includes the cutting of slabs of germaniumof approximately .03 inch thickness, the germanium being appropriatelydoped and polished with 600A grit metallographic paper and being swabbedwith cotton dipped in acetone.

The polished surfaces of the slabs are etched in order to exposesurfaces of the germanium having desirable electrical characteristics.An effective illustrative bath includes approximately 36 grams of sodiumperoxide per hundred milliliters of water. The sodium peroxide issprinkled into a container of water which holds the germanium wafers tobe etched. There is an active bubbling that takes place, and thesolution is self-heating, rising to about 45 C. The bubbling subsidesand the treat ment is completed in about ten minutes. The concentrationis subject to appreciable variation, 25 to 40 grams per millilitersbeing effective. The temperature of the reaction seems not to becritical, but either cooling or heating appears to be detrimental tobest results. In place of the sodium peroxide solution, a bath of equalparts of 25% sodium hydroxide in water and 30% hydrogen peroxide inwater may be used efiectively to etch germanium, where the sodium shouldperferably be added first whereupon the solution heats itself and thehydrogen peroxide is then added. Three to four minutes is effective foretching the germanium.

Subsequent to etching for appropriate periods, the ingredients used areflushed away by a water wash, and/or volatile organic liquids.

Diodes using germanium prepared in this manner may be completed bysupporting the germanium with the prepared surface engaged by one ormore suitable rectifying contacts, in accordance with a range of wellknown forms of construction.

What is claimed is:

I. The method of etching a germanium crystal including the steps ofimmersing the crystal in an alkaline water solution of an alkali-metalperoxide.

2. The method of etching a germanium crystal including the steps ofimmersing the crystal in an alkaline water solution of an alkali-metalperoxide of the group consisting of sodium peroxide and potassiumperoxide.

3. The process of preparing a surface of a body of germanium for use ina semiconductor translator which includes etching said surface in analkaline aqueous solution of sodium peroxide.

4. The process of preparing a surface of a body of germanium for use ina semiconductor translator which includes etching said surface in analkaline aqueous solution of potassium peroxide.

5. The method of preparing a surface of a germanium body for use in asemiconductor translator which includes immersing said surface in analkaline solution of hydrogen peroxide and an alkali-metal hydroxide.

6. The method of preparing a surface of a germanium body for use in asemiconductor translator which includes immersing said surface in analkaline solution of hydrogen peroxide and an alkali-metal hydroxide ofthe group consisting of sodium hydroxide and potassium hydroxide.

7. The method of preparing a surface of a germanium body for use in asemiconductor translator which includes immersing said surface in analkaline aqueous solution of hydrogen peroxide and sodium hydroxide.

8. The method of preparing a surface of a germanium body for use in asemiconductor translator which includes immersing said surface in analkaline aqueous solution of hydrogen peroxide and potassium hydroxide.

9. The method of making germanium crystal translators including thesteps of cutting a doped ingot of germanium into slabs, and etching acut surface of a slab in an alkaline aqueous solution of an alkali metalperoxide.

10. The method of making germanium crystal translators including thesteps of cutting :1 doped ingot of germaniurn into slabs, and etching acut surface of a slab in an alkaline aqueous solution of an alkali metalperoxide of the group consisting of sodium peroxide and potassiumperoxide.

ll. The method of etching crystalline germanium including the step ofimmersing a specimen of germanium in an alkaline aqueous solutioncontaining approximately 36 grams per hundred milliliters of sodiumperoxide.

l2. The method of making a germanium crystal translator which includesthe steps of etching a specimen of crystaiiine germanium in an alkalineaqueous solution of an alkali-metal peroxide of the group consisting ofsodium peroxide and potassium peroxide, and assembling the etchedspecimen in engagement with a rectifying T contact.

References Cited in the file of this patent UNITED STATES PATENTS2,577,803 Pfann Dec. 11, 1951 2,588,008 Jones et al Mar. 4, 19522,619,414 Heidenreich Nov. 25, 1952

1. THE METHOD OF ETCHING A GERMANIUM CRYSTAL INCLUDING THE STEPS OFIMMERSING THE CRYSTAL IN AN ALKALINE WATER SOLUTION OF AN ALKALI-METALPEROXIDE.